Subarctic Wildfire Activity Is Heating Up

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Subarctic wildfire frequency is higher now than it has been at
any other point in the last 10,000 years, new records show.

The records, obtained from charcoal in the Yukon Flats of Alaska,
have revealed the history of wildfire activity in the region
known as the subarctic, the area just south of the Arctic Circle,
from North America to Scandinavia and Siberia, where boreal
forests dominate and winters are long and dark.

But what the higher frequency of wildfires spells for the
subarctic in a warmer future world is difficult to predict,
researchers say.

"The climate is predicted to get warmer, and this
favors more fires," said Ryan Kelly, a plant biologist at the
University of Illinois who examined the records. On the other
hand, the spike in wildfires is transforming Alaska's coniferous
forests into woodlands made up of relatively fire-resistant
deciduous trees. In the past, this shift resulted in a kind of
vegetative feedback that put the breaks on additional burnings.

"Will there be a feedback now?" Kelly said. "Maybe. That fits
with what happened before."

The new study, detailed in the July 22 issue of the journal
Proceedings of the National Academy of Sciences, is part of
decades-long research seeking to understand how ecosystems have
changed in the past, Kelly told LiveScience, adding that this
information will help scientists predict what may happen in the
future. The current project focused on boreal forests, which make
up about 10 percent of the Earth's land surface and more than 30
percent of its terrestrial carbon stock. [ Earth
in the Balance: 7 Crucial Tipping Points ]

For the study, Kelly and his colleagues investigated the past
fire regimes of the Yukon Flats, a region whose recent fire
activity is one of the highest among North American boreal
forests. The team
collected ancient charcoal samples buried in the mud at the
bottom of 14 lakes in the area.

By analyzing the charcoal, the researchers could tell when
individual wildfire events occurred, and how severe they were.
(The amount of charcoal present indicates how much of the forest
burned.) By collecting samples of pollen in the sediment, they
could also tell which tree species populated the Yukon Flats over
time.

The scientists discovered that fire frequency and severity
increased during the period between 6,000 and 3,000 years ago,
coinciding with the spread of the highly flammable black spruce
(Picea mariana). During the
Medieval Climate Anomaly (MCA) between 1,000 and 500 years
ago, the warm and dry climatic conditions — which are most
similar to today's, compared with other periods in the
10,000-year record — caused intense fires to pop up.

However, the severe wildfires promoted the growth of
fire-resistant plant species, resulting in a gradual shift from
conifer-rich boreal forests to deciduous woodlands. Aspens
(Populus tremuloides) and other deciduous trees limited
the frequency of fires and prevented it from increasing more than
in previous periods.

"Normally, you'd expect that if there is more burning overall in
the region, that probably means there is a larger number of
fires," Kelly said. "But that's not what we found."

Present and future fire

In the last 50 years, the Yukon Flats have seen the same kind of
changes in tree composition that occurred during the MCA, with
young
deciduous trees slowly taking over the territory once occupied
by black-spruce trees. But the wildfire activity there has
already surpassed the MCA limit: The current fire frequency in
the flats is about 20 wildfire events per 1,000 years, compared
with the previous rate of about 10 events per 1,000 years.

Humans appear to be the culprits behind this increase in severe
wildfires.

"It's not that people are starting the fires, but there's a
pretty clear link between humans inducing a warmer climate
and increased forest burning," Kelly said, adding that lightning
can more readily spark a wildfire when the environment is dried
out from high temperatures.

The results are particularly striking, given that wildfires
release the forests' stored carbon into the atmosphere,
potentially exacerbating climate change. And it's plausible that
as the climate continues to warm, even the fire-resistant
deciduous trees could start to burn.

"We may be out of the realm of what has happened in the past,"
Kelly said. "Are we now entering into something totally
different?"